In an optical exposer for projecting and transferring a predetermined pattern onto a work piece through a projector lens, there prevails a superposed exposure whereby a plurality of predetermined patterns are projected successively and so transferred onto the work piece. However, such superposed exposure calls for strict precision hence, a correcting or realignment must be carried out.
For this positional correction, a common method employed is a technique wherein an alignment mark is formed on the work piece, and the alignment mark is detected through a projector lens. With this technique, the alignment mark is detected as an electrical signal waveform, therefore a position of the alignment mark is determined by the electrical signal waveform.
However, according to the inventor's experimental research, the electrical signal waveform detected is not actually a true electrical signal waveform representation of the alignment mark; consequently, it has been found that an accurate positioning of the alignment mark cannot be decided easily. As a result of the inventor having further studied the cause, it has been found that a projector lens is normally designed so as to use a monochromatic light only for enhancing the resolving power of the exposure; consequently, said monochromatic light must be used also as the light for detecting the alignment mark; therefore, there is distortion of the light due to the non-uniform thickness of a resist film formed on the surface of the work piece, the thickness of which is roughly the same as the wavelength of the monochromatic light; thus, the electrical signal waveform of the alignment is distorted to a considerable degree.